This function shoots a shot unless a shot is in the air. The timerCount is reset to zero. The shootAngle and shootForce variables are set to the current cannon angle and force. Finally, we start the timer.

moveShot() is the slot that moves the shot, called every 5 milliseconds when the Qt::Timer fires.

Its tasks are to compute the new position, update the screen with the shot in the new position, and if necessary, stop the timer.

First we make a Qt::Region that holds the old shotRect(). A Qt::Region is capable of holding any sort of region, and we'll use it here to simplify the painting. shotRect() returns the rectangle where the shot is now. It is explained in detail later.

Then we increment the timerCount, which has the effect of moving the shot one step along its trajectory.

Next we fetch the new shot rectangle.

If the shot has moved beyond the right or bottom edge of the widget we stop the timer, or we add the new shotRect() to the Qt::Region.

Finally, we repaint the Qt::Region. This will send a single paint event for just the one or two rectangles that need updating.

This private function calculates the center point of the shot and returns the enclosing rectangle of the shot. It uses the initial cannon force and angle in addition to timerCount, which increases as time passes.

The formula used is the standard Newtonian formula for frictionless movement in a gravity field. For simplicity, we've chosen to disregard any Einsteinian effects.

We calculate the center point in a coordinate system where y coordinates increase upward. After we have calculated the center point, we construct a Qt::Rect with size 6 x 6 and move its center point to the point calculated above. In the same operation we convert the point into the widget's coordinate system (see The Coordinate System).